The present invention relates to a plasma processing apparatus for surface treatment to etch a substrate or to form a thin film with a plasma by supplying a radio-frequency electric field to an antenna, generating an electric field, and thereby generating a plasma by the electric field, and a method of using this apparatus. More particularly, the invention relates to a semiconductor processing apparatus for processing a semiconductor device, and a method of using this apparatus.
In a semiconductor processing apparatus for generating a plasma by induction by feeding an electric current to a coil-shaped antenna, there is a problem that a vacuum chamber wall made of an non-conductive material and enclosing a plasma generating unit so as to establish a vacuum atmosphere is partly removed by the plasma. In order to solve this problem, there has been conceived a method using a field called the xe2x80x9cFaraday shieldxe2x80x9d, as disclosed in Japanese Patent Laid-Open No. 502971/1993. If the Faraday shield is used, however, the plasma ignitability is so deteriorated that the plasma is not ignited unless a voltage as high as tens of KV is applied to the feeding portion of the coil-shaped antenna. This apparatus may fail with a high possibility by the discharge between the antenna and a conductive structure nearby. In order to prevent this discharge, an additional structure is needed to insulate the antenna from the existing structure, causing the apparatus to be complicated.
When a Faraday shield is used to reduce the partial removal of the wall, foreign matters are liable to adhere to the wall and to appear if its sticking rate to the wall from the plasma is accelerated. Therefore the partial removal of the wall must be adjusted according to the process.
The plasma density distribution is determined mainly by the generation rate distribution and by the state of transportation of ions and electrons. In the absence of an external magnetic field, the transportation of the plasma diffuses isotropically in every direction. At this time, electrons instantly escape and tend to reach the wall of the vacuum chamber because the mass is no more than 1/1,000 of that of an ion, but they are repelled by the sheath (ion sheath) formed in the vicinity of the wall. As a result, a quasi-neutral condition of the electron and ion densities is always met in the plasma, so that both the ions and electrons are bipolarly diffused toward the wail. At this time, the potential of the plasma takes on its maximum where the plasma density, i.e., the ion density, is the maximum. This potential is termed the plasma potential Vp, approximately expressed by Vp≈T≈exc3x97ln(mi/me), where Te, mi and me are the electron temperature, the mass of an ion, and the mass of an electron, respectively. In the plasma, the potential distribution is determined by the potential Vp and the wall potential (ordinally at 0 V), so that the density distribution is correspondingly determined. Since, in this case, the plasma is confined by the electrostatic field established by itself, the density distribution is determined by the shape of the apparatus, the place where the induced electric field takes on the maximum, and the ratio of the generation rate/the bipolar diffusion flux.
When the coil is wound by several turns on the vacuum chamber, for example, the magnetic flux generated by the coil takes on the maximum at the central portion so that the induced electric field takes on the maximum at the central portion. Moreover, the induced electric field cannot penetrate deeper than about the skin depth, e.g., 1 cm, so that both the ionization factor and the dissociation factor take on their maximums at the radially central portion (in the direction of arrow r, e.g., in FIG. 21(a)) and just below the dielectric member (in the direction of arrow z, e.g., in FIG. 21(a)). After this, the plasma diffuses towards the wafer side (downstream side). In the case of an ordinary chamber having a cylindrical shape, therefore, the plasma density is the maximum at the central portion in the direction of arrow r, and the degree of central concentration rises downstream so that the plasma density becomes nonuniform in the region where the wafer is placed.
A first object of the invention is to control the removal extent of the vacuum chamber wall around the plasma generating portion by the plasma. A second object of the invention is to improve the plasma ignitability.
A third object is to realize a uniform plasma of high density. This object is particularly desired in processing large semiconductor wafers (e.g., large-size semiconductor wafers of 300 mm).
In order to achieve the above-specified objects, according to the invention, there is provided a plasma processing apparatus comprising an antenna (coil) for generating an electric field in a plasma generating portion, a radio-frequency power source for supplying radio-frequency electric power to said antenna, a vacuum chamber enclosing the plasma generating portion to establish a vacuum atmosphere therein, a Faraday shield provided around said plasma generating portion (e.g., around the vacuum chamber), a gas supply unit for supplying gas into said vacuum chamber, a sample stage on which an object to be processed is placed, within the vacuum chamber, and a radio-frequency power source for applying a radio-frequency electric field to said sample stage, a plasma being generated by accelerating electrons and ionizing them by collision with the electric field generated by said antenna, and thereby processing said object; characterized in that a load is provided in the earth portion of said antenna, the average potential of said antenna is adjusted so as to improve the ignitability at a plasma ignition time, and the load is adjusted after the plasma is produced so that the average potential of said antenna may be close to that of the earth, and the removed amount of the wall of said vacuum chamber after the plasma generation may be small. The above-specified objects are also achieved, according to the present invention, by a method of operation of this apparatus whereby the load provided in the earth portion of the antenna is adjusted (that is, the voltage on the ends of the antenna (coil) is controlled) such that ignitability of the plasma at the time of plasma ignition is facilitated, and is then again adjusted to be close to that of the ground to limit the amount of chamber wall removed (e.g., etched) by the plasma.
Here, the phenomenon that the average potential of the antenna comes close to that of the earth means that the potentials 30a and 30b of FIG. 4 are mutually opposite in phase but substantially equal to each other, that is, Va≈xe2x88x92Vb.
As another technique and structure to achieve the above-specified objects, the Faraday shield can be provided with at least one switch. When igniting the plasma at a plasma ignition time, the at least one switch is positioned such that the Faraday shield is held in a floating state, to facilitate ignition of the plasma. Thereafter, the at least one switch is thrown to ground the Faraday shield, so as to protect the wall of the plasma chamber from removal by the plasma.
As still another technique and structure to achieve the above-specified objects, the load can be provided in the earth portion of the antenna and a switch or switches can be provided for the Faraday shield. By adjusting the load and positioning the switch as described in the preceding paragraphs, ignition of the plasma is facilitated and removal of the wall in the plasma chamber is avoided.
Means for solving the above-specified problems will be described with reference to FIG. 2. FIG. 2 shows an experimental induction type plasma generating apparatus, used for verifying the present invention. With this apparatus, the methods for reducing the partial removal of the vacuum chamber wall around the plasma generating portion by the plasma and for improving the ignitability of the plasma are examined by changing the way of grounding the Faraday shield and the antenna to the earth.
In this apparatus, a mixed gas of a chlorine gas and a boron trichloride gas is supplied into a vacuum chamber 2 made of alumina, by the gas supply unit 4. The gas is ionized to produce a plasma 6 with the electric field which is generated by a coil-shaped antenna 1 of two turns wound around the vacuum chamber 2. After this plasma production, the gas is discharged to the outside of the vacuum chamber by a discharge unit 7. The electric field for producing the plasma is generated by feeding the antenna 1 with radio-frequency electric power of 13.56 MHz generated by a radio-frequency power source 10. In order to suppress the reflection of the electric power, an impedance matching unit 3 is used to match the impedance of the antenna 1 with the output impedance of the radio-frequency power source 10. The impedance matching unit is one using two capacitors of variable capacitance, generally called an xe2x80x9cinverted L typexe2x80x9d. The other end of the antenna is grounded through a capacitor 9 to the earth, and a switch 21 is provided for shorting the capacitor 9. In order to prevent the vacuum chamber 2 from being etched by the plasma 6, moreover, a Faraday shield 8 is interposed between the antenna 1 and the vacuum chamber 2. By turning on/off a plurality of switches 22, the Faraday shield can be brought into either the grounded state or the ungrounded state. FIG. 3 is a perspective view showing the state that the Faraday shield is installed. This Faraday shield 8 is provided with a slit 14 for transmitting the inductive electric field 15a generated by the coil-shaped antenna 1, into the vacuum chamber but intercepting a capacitive electric field 15b. The plasma is ignited mainly with the capacitive electric field 15b. When the Faraday shield is grounded to the earth, however, the capacitive electric field from the antenna is hardly transmitted into the vacuum chamber, thereby deteriorating the ignitability of the plasma. When the Faraday shield is not grounded to the earth, the antenna and the Faraday shield are capacitively coupled to bring the potential of the Faraday shield close to the average potential of the antenna. Thus, it is considered that the capacitive electric field is established between the Faraday shield 8 and an electrode 5, and hence the ignitability of the plasma is not deteriorated so much.
The capacitive electric field 15b is normal to the wall of the vacuum chamber 2, so that the charged particles in the plasma are accelerated to impinge upon and damage the wall. Light 16 emitted from the plasma was observed with a spectroscope 20, and the removal of the wall was measured by observing the light emission strength of aluminum in the plasma as the wall aluminum was removed.
First of all, here will be described a method for optimizing the capacitance of the capacitor 9 connected to the earth portion of the antenna in the experimental apparatus shown in FIG. 2 so that the removal of the wall may be reduced. In the following, the conduction state between the two ends of the switch will be referred to as xe2x80x9conxe2x80x9d, and the cut-off state will be referred to as xe2x80x9coffxe2x80x9d. With the switch 21 being off, that is, with the capacitor 9 being not shorted, here will be described the optimum value of the magnitude of the capacitance of the capacitor 9. The experimental apparatus of FIG. 2 can be shown as an equivalent circuit in FIG. 4. Then, the antenna 1 acts as the primary coil of a transformer, and the plasma 6 acts as the secondary coil of the same. The antenna 1 and the plasma 6 are coupled capacitively, and their capacitance is shown by capacitors 31a and 31b. The capacitance C of the capacitor 9 is determined so that a relation of Va=xe2x88x92Vb always holds between the potential Va at the position of the point 30a on the circuit and the potential Vb at the position of the point 30b when the antenna has an inductance L. When this condition is satisfied, the potentials to be applied to the two ends of the capacitors 31a and 31b are minimized, minimizing the wall damage. FIG. 5 further simplifies the circuit of FIG. 4, namely the antenna and the plasma are combined together as an element 17 having one combined impedance. The impedance of the element was experimentally determined to be Z1=2.4+141j(xcexa9), where j is a complex number. This measurement of the impedance can be simply executed by measuring the electric current flowing through an object to be measured, and the voltages at the two ends of the object. The capacitor 9 has an impedance Z2=xe2x88x92(1xe2x88x92xcfx89C)j, where xcfx89 is the angular frequency corresponding to 13.56 MHz. For Va=xe2x88x92Vb, the relation between the impedances Z1, Z2 is (Z1+Z2):Z2=1:xe2x88x921 since the real part of Z1 is so small that it can be ignored. The calculated electric capacitance of the capacitor 9 is about 150 pF, therefore, the relation Va=xe2x88x92Vb holds. FIG. 6 illustrates the results of calculation of the amplitudes of the potentials at the point 30a (the dotted curve) and the point 30b (the solid curve). The graph shows the capacitance of the capacitor 9 as the abscissa, and the amplitudes of the generated potentials as the ordinates. As a result, the generated potentials were mutually equal in the vicinity of the capacitance of 150 pF of the capacitor 9, the phases of the oscillating voltages at that time were shifted by 180 degrees, and the relation of Va=xe2x88x92Vb was satisfied. This makes it possible to determine by the method thus far described such a capacitance of the capacitor to be connected to the earth side of the antenna that the damage of the wall is minimized.
Next, with the capacitance of the capacitor 9 fixed at 150 pF in FIG. 2, the removed amount of the wall and the plasma ignitability were examined, as tabulated in FIG. 15, when the switches 21 and 22 are turned on or off. The wall removal is found to be great when the switch 21 is on and the switch 22 is off. Under this condition, the plasma ignitability is excellent. Under the other conditions, however, the wall removal can be reduced, but the plasma ignitability is low. Therefore, it has been found that the condition for little wall removal and for excellent plasma ignitability is not present in this system. However, these two purposes can be achieved by operating either the switch 21 or the switch 22 so as to reduce the wall damage after the plasma was ignited under the condition that the switch 21 is on and the switch 22 is off at the ignition time. Here, it is better to use only the switch 21 for the simplification of the apparatus structure. This is partly because the potential of the Faraday shield has to be lowered to zero as much as possible so as to reduce the wall damage by using the switch 22, and consequently the switch 22 has to be provided in plurality, and partly because the Faraday shield has to be grounded with the shortest distance to the earth so that the plural switches 22 have to be provided just near the antenna and the Faraday shield. If the plural switches are arranged for those necessities at the portion adjacent to the antenna and the Faraday shield, the result is a complicated structure. This complicated structure can be avoided with respect to the switch 21 because only one switch 21 is connected to the capacitor 9 side which is provided at a considerable distance from the antenna.
The off state of the switch 21 is the state that the capacitor of 150 pF is connected between the antenna and the earth, and the on state of the switch 21 is identical to the state that the capacitance of the capacitor 9 is increased to infinity in a radio-frequency band of HF or VHF. This means that the wall removal increases more as the capacitance of the capacitor 9 is raised to a higher level from 150 pF. The wall removal also increases even if the capacitance of the capacitor 9 is lowered from 150 pF. Thus, the wall removal can be controlled by varying the capacitance of the capacitor 9.
In an apparatus shown in FIG. 7, the capacitance of the capacitor 9 connected to the earth side of the antenna 1 is variable, so that the wall removal by the plasma can be reduced by varying the capacitance of the capacitor 9. Moreover, the plasma ignitability can be drastically improved by making the capacitance of the capacitor 9 far larger or smaller than 150 pF at the time of igniting the plasma.
By adjusting the capacitance of the capacitor connected to the earth side of the antenna, as described above, the removed amount of the wall by the plasma can be reduced to achieve the first object of the invention. At the plasma ignition time, moreover, the capacitance of the capacitor connected to the earth side of the antenna can be changed to establish an excellent ignitable state, thereby achieving the second object of the invention.
Here will be examined a method for generating a uniform plasma. When the coil-shaped antenna is placed on the upper face of the vacuum chamber, the induced electric field is generated at the central portion, even if the diameter of the antenna is varied to vary the intensity of the induced electric field in the radial direction, so that the plasma density distribution is nonuniformly concentrated at the center. This tendency of concentration of the plasma density at the center is not varied even if a plurality of antennas are arranged to vary the distance between each antenna and the dielectric member. FIG. 21(b) illustrates one example of the calculation of the plasma density distribution when the antenna is placed on the vacuum chamber like FIG. 21(a). From this calculation, when the ratio of the apparatus height H to the radius R (the aspect ratio) is as large as H/R=20/25, as illustrated in FIG. 21(b), the plasma density at the place, just below the antenna (z=2 cm), where the antenna is present takes on its maximum and increases in its absolute value (z=10 cm) downstream (in the direction where the value z increases) but is small just above the substrate. It is then found that the plasma density is nonuniform. When viewed in the z direction, the density takes on its maximum at the apparatus center z=10 cm. When the aspect ratio is reduced, as illustrated in FIG. 21(c), the density distribution is substantially identical to that of FIG. 21(b), but the distribution just above the substrate is gentler than that of (b) and is concentrated at the center.
The plasma density distribution is determined by the boundary condition that the plasma density is zero on the vacuum chamber wall and by the generation rate distribution, i.e., the antenna position. Even if the antenna position is changed, as illustrated in FIG. 21(d), and if a plurality of antennas are placed to change the power distribution, the shape of the density distribution remains unchanged. When the coil is provided on the upper face, the induced electric field generated by the antenna takes on its maximum just below the antenna, so that a centrally concentrated distribution is always established on the downstream.
In the case of an arrangement in which the antenna is wound horizontally around the vacuum chamber, the induced electric field takes on its maximum on the side face of the chamber. A sheath is formed on the side face of the chamber, so that the plasma density takes on its maximum slightly inside the sheath, at the place the closest to the antenna. As shown in a horizontal section at this time, the potential is higher at the sheath end than at the wall and than at the plasma center so that the plasma is transported to the two sides from the sheath. Simultaneously with this, the plasma flows downstream from that position, and hence the density distribution is uniform in a portion in a horizontal section, at a distance in the z direction from the highest density portion. In the case of a cylindrical apparatus, for example, a concave distribution may be established in the vicinity of the wafer for a small H/R ratio, and a convex distribution may be established for a sufficiently large H/R ratio where H is the height of the apparatus and R is the radius thereof, so that the plasma density distribution can be controlled to some extent (refer to FIGS. 22(a) and 22(b)). The dominant factors at this time are the shape of the apparatus, i.e., the ratio H/R. When the antenna is provided on the side face, however, the plasma density is lowered by the reduction in the coupling efficiency due to the large coupling area of the antenna and the plasma and by the large loss of the plasma because the region where the density is a maximum is near to the side face wall. If the supplied power and the vacuum chamber size are the same, the plasma density of this case is lower than that of the aforementioned case in which the antenna is provided on the upper face. This raises a problem that the processing speed of the object to be processed is low.
As thus far described, the plasma density distribution of the inductively coupled plasma varies with the apparatus shape and the antenna arrangement, but the third object of the invention is achieved by such a construction where the upper face of the vacuum chamber has a smaller area than that of the lower face, and the upper face is flat. Thus, the apparatus of the present invention can be used to process large-sized semiconductor wafers discussed previously.
In the plasma processing apparatus, preferably, the angle between the edge at which the lower face and the upper face intersect and the normal of the upper face is not less than 5 degrees.
In the plasma processing apparatus, more preferably, the ratio of the apparatus height (the distance from the object to be processed to the upper face) to the lower surface radius Rd is not more than 1.